compute_ctl: return LSN in /terminate (#12240)

- Add optional `?mode=fast|immediate` to `/terminate`, `fast` is
default. Immediate avoids waiting 30
  seconds before returning from `terminate`.
- Add `TerminateMode` to `ComputeStatus::TerminationPending`
- Use `/terminate?mode=immediate` in `neon_local` instead of `pg_ctl
stop` for `test_replica_promotes`.
- Change `test_replica_promotes` to check returned LSN
- Annotate `finish_sync_safekeepers` as `noreturn`.

https://github.com/neondatabase/cloud/issues/29807

safekeeper/tests/walproposer_sim/walproposer_api.rs

@@ -499,7 +499,7 @@ impl ApiImpl for SimulationApi {
         true
     }
 
-    fn finish_sync_safekeepers(&self, lsn: u64) {
+    fn finish_sync_safekeepers(&self, lsn: u64) -> ! {
         debug!("finish_sync_safekeepers, lsn={}", lsn);
         executor::exit(0, Lsn(lsn).to_string());
     }